Facsimile recording amplifier



FIG.I

G. B. WORTHEN 2,794,913

FACS IMILE RECORDING AMPLIFIER Original Filed Nov. 16, 1948 June 4, 19573 Sheets-Sheet 1 TO RECORDING STYLUS RADIO RECEIVER INVENTOR.

G. B. WORTH E N wdzwk- ATTORNEY e. B. WORTHEN 2,794,913

FACSIMILE RECORD/INC AMPLIFIER June 4, 1957 Original Filed Nov. 16, 19483 Sheets-Sheet 2 FIG. 2

TUBE 327 TUBE 325 TUBE 326 OUTPUT OUTPUT AMPLIFIED OUTPUT 0F WHOLE WAVEWEAK INPUT LEVEL WI- IERE POSITIVE SIGNAL INPUT VOLTAGE EQUALS NEGATIVEBIAS VOLTAGE AND TUBE STARTS TO CONDUCT.

NEGATIVE PULSE SLIGHTLY COMPRESSED SMALL PORTION OF POSITIVE PEAKCOMPRESSED (DUE TO RESISTOR 325a) AMPLIFIED OUTPUT NEGATIVE PULSE PARTLY COM PRE SSED MODERATE INPUT SIGNAL STRONG INPUT SIGNAL PORTION OFPEAK COMPRESSED POSITIVE PULSE REDUCED OUT PUT NEGATIVE PULSE ELIMINATEDINVENTOR.

G. B. WORTHEN ATTORNEY 49am 2 77 y- June 4, 1957 e. B. WORTHEN FACSIMILERECORDING AMPLIFIER Original Filed Nov. 16, 1948 3 Sheets-Sheet 3 FIG. 3

OUTPUT OF omrznaurmrmc v cmcun 32a= 32w TUBE 32a LEFT SIDE RIGHT SIDEINVENTOR.

G. B. WORTHEN ATTORNEY United States Patent FACSIMILE RECORDINGAMPLIFIER George B. Worthen, New York, N. Y., assignor to The WesternUnion Telegraph Company, New York, N. Y., a corporation of New YorkOriginal application November 16, 1948, Serial No. 60,334. Divided andthis application January 8, 1953, Serial No. 330,341

2 Claims. (Cl. 25027) This invention relates to the art of amplifyingreceived radio signals and more particularly to an amplifier for afacsimile recorder.

This application is a division of applicants copending applicationSerial No. 60,334, filed November 16, 1948, now Patent No. 2,647,945.The latter application discloses a facsimile system including a portabletransmitter communicating with a distant recorder. The instant inventionis concerned with the novel recording amplifier, and only so much of thesystem disclosed in the parent application as is necessary for anunderstanding of the present invention is herein disclosed.

In the recording of facsimile messages there are no intermediateintelligence signals. The recorder is con: cerned only with black andwhite indications, that is, signal and no signal conditions. However,for good reproduction of the transmitted message, each signal must beapproximately equal in energy content, in order to reproduce spots ofconstant density. Also, the rejection of noise signals during a nosignal interval is highly desirable to avoid recording a spot where awhite area should appear.

Accordingly, applicant has devised a facsimile recording amplifier withthe primary objective of reshaping both strong and weak input signalsinto substantially uniform waves for recording.

A further object of this invention is to provide a recording amplifierthat will reject substantially all intermediate noise signals.

The invention will be more fully understood by reference to thefollowing description taken with the drawings in which:

Fig. 1 is a diagram of the recording amplifier;

Figs. 2 and 3 consist of wave diagrams which indicate roughly theprogress of signal waves of different levels through the recordingamplifier.

To facilitate reference to the above-cited parent ap plication, thevarious elements of the drawings are given the same reference numerals.

The recording amplifier utilizes five vacuum tubes numbered 325 to 329.The characteristics and interstage connections of these tubes are suchthat the amplifier not only amplifies the received signals but alsoperforms the automatic functions of power level control, noiserejection, signal wave shaping and the uniform limiting of signal outputwith a wide variation of input level. The first three tubes 325, 326,327 are pentodes with a novel form of coupling and operation to bepresently described. Tubes 325 and 326 have a remote cut-01f, while tube327 has a sharp cut-off. The fourth tube 328 of the amplifier is a dualtriode which operates as a limiter and wave shaper of signals over awide range of input level and frequencies. The final tube 329 is abeam-power tube connected to the output transformer 330 which suppliesthe recording voltages to the stylus. The plate circuits of all thetubes 325 to 329 are connected to the 250 volt Patented June 4, 1957 2D. C. line 359. The conductor 360 represents the com mon ground of thetubes.

At the input end of the recording amplifier is a jack 361 into which isplugged the shielded output cable 362 5 of a portable radio receiver363. The spring contact of jack 361 is connected to the control grid 364of tube 325, which is therefore operated by the input voltages picked upby the radio receiver. These voltages comprise not only the facsimilesignal voltages sent out by a radio transmitter but also the lower noisevoltages which modify the signal wave. The circuit connections betweenjack 361 and the control grid 364 include a D. C. blocking condenser325a, a resistor 352b, a grid bias condenser 325C, and a high resistor325d which controls the discharging time of the bias condenser. A loadresistor 3252 constitutes the input impedance of the amplifier.

Resistors 325] and 325g determine the value of the voltage impressed onthe screen grid 364a of tube 325 from the supply conductor 359 which inthis case is at +250 volts and which also supplies power to the plate3641: of the tube across a resistor 32512. It is important to point outthat in this particular application of tube 325 the voltage on screengrid 364a is of low value as compared with the plate voltage. Forexample, in the present embodiment of our invention the resistors 325]and 3255 are of such values as to fix the voltage on screen grid 364a at1 or 2 volts positive. This low screen voltage and the value of resistor32511 are the chief factors that so determine or adjust the operatingcharacten istics of tube 325 as to make its cut-off more gradual andtherefore cause the recording amplifier to work with a wide variation ofinput signal strength.

As described in the cited parent application, a phasing pulse is sentout by a radio transmitter at each revolution of a scanning drum and asignal pulse is transmitted every time a stylus scans a black mark onthe message copy. Each phasing pulse and scanning signal picked up bythe radio receiver 363 charges the condenser 325a across the lowresistor 325b very rapidly to the maximum amount for the particularsignal level of the moment. This condenser charge is impressed on thecontrol grid 364 of tube 325 as a negative bias which determines theamplification of the tube.

On weak signals the charge on condenser 3250 is small and therefore thenegative bias on grid 364 is low, whereby the amplification of tube 325is correspondingly high. When strong signals are received, the charge oncondenser 3250 is high and the resulting high negative grid bias keepsthe amplification of the tube down. In consequence, the tube 325amplifies weak signals to a greater degree than strong signals, so thatthe output of the tube is fairly constant for a wide variation of signalinput. For weak signals the tube 325 amplifies the whole wave, with thenegative pulse slightly compressed. For moderate and strong signals thetube 325 compresses progressively more of the negative pulses withincreasing signal strength. This action of tube 325 on weak, moderateand strong signals is illustrated in diagrams A1, B1 and C1 of Fig. 2.

While the charging time of the grid bias condenser 3250 is very short,the rate of discharge is comparatively low due to the high value ofresistor 325d. In the present form of our system as herein set forth,the charging time of condenser 325a is less than the duration of aphasing pulse sent out by the transmitter, and the discharging timeendures for at least one revolution of the transmitter dr'um (that is,for one scanning line). For this purpose we use a condenser of largecapacity (say, 0.1 mfd.) and make the resistor 325d of high value (say,10 megohms). These figures are mentioned merely by way of example.

With the charging time of condenser 3250 less than the I duration of aphasing pulse, it is certain that the condenser charge will always reachfull value during a phasing pulse at any signal level. In this case itis necessary to make the discharging time of condenser 3250 cover atleast one turn of the transmitter drum for the following reason: As wehave seen, a phasing pulse is sent to the recorder once for eachrevolution of the transmitter drum for a brief duration of time and thispulse places a negative bias voltage on grid 364 of tube 325. Now, iffor the remainder of a scanned line no black marks are encountered bythe stylus, there is an absence of signals and (without specialprovisions) the bias voltage on grid 364 would decrease, theamplification of tube 325 would increase, and any noise present would bepassed on for recording. To prevent this from happening we make thedischarge time of condenser 325a long enough to keep sufficient negativebias on grid 364 for one scanning line, so that the amplification oftube 325 is held down to a point where noise signals will not pass.

The elements 325b, 325s and 325d constitute a grid leak combinationwhich determines the bias on grid 364 of tube 325. Since this tube is ofthe remote cut-off type, its amplification varies with the effectivegrid bias, which in turn varies with the strength of the input signals.Ihe time constant of the grid leak combination 325b'325c- 325d is suchthat the bias on grid 364 when once established decreases rather slowlybut increases rapidly. As the signal increases, the current flowing tothe grid 364 increases correspondingly and thus charges the condenser3250, so that a negative bias is applied to the grid by the voltage dropacross the resistor 325d.

The operating characteristics of tube 325 (as determined by the slope ofits transfer characteristic curve) are such that strong or large signalsare amplified proportionately less than weak signals. As indicated indiagram B1 and C1 of Fig. 2, the positive peaks of increasing signalsare compressed by the resistor 325b which acts as a limiter to dissipateexcess voltage and thereby prevent severe noises from varying the signaloutput of tube 325. :The resistor 325b performs the additional functionof slowing down the charging time of condenser 3250. This preventssudden strong bursts of local static from interfering with the passageof normal signals which will record as soon as the D. C. bias appliedmomentarily to condenser 325s by the static burst has drained off.

It is a characteristic of tube 325 that it operates with a signalrejection rate which increases with increase of signal. This isindicated in diagrams B1 and C1 of Fig. 2 by the horizontal lines x andx which represent each the time interval between the beginning of acycle and the instant when plate current flows through the tube. For allbut the weaker signals, tube 325 rejects the incoming wave until thesignal voltage rises to a point where the plate draws current. That isto say, the tube 325 rejects any signal voltage lower than the grid biasvoltage and the interval during which the signal voltage rises to thelevel of the bias voltage is represented by the lines x and x in thewave diagrams. For the purpose of description we may regard the lengthsof lines x and x which increase with the rise in signal level, asdenoting roughly the signal rejection characteristics of tube 325.

The output of tube 325 is passed to the second tube 326, which operatesas a limiter for all but the Weakest signals which are slightlyamplified. The tube 326 operates with a low negative bias (say, minus0.5 volt) and with very low plate and screen voltages. The plate voltage(6 to 8 volts positive) is determined by the value of resistor 326e andthe screen voltage is determined by the voltage divider consist-ing ofresistors 3261 and 326g, these figures being given merely as practicalexamples. The signals coming from tube 325 charge the condenser 326cwhich applies a negative bias to the control grid 326a across a resistor32Gb. The condenser 3260 is connected in series with a resistor 326d toform a grid limitin'g circuit which has a much more rapid time constant4 than the grid leak circuit of tube 325. Consequently the tube 326passes on the more positive portion of the signal wave while rejectingthe negative portion. As the signals entering tube 326 increase inlevel, they are increasingly more limited because of the value ofresistor 326b and the characteristics of the tube. The effect of tube326 on weak, moderate and strong signals is illustrated in diagrams A2,B2 and C2 of Fig. 2.

The characteristics of tube 326 include the function of decreasing thesignal rejection rate with increasing signal level. This, it will benoted, is the reverse of the rejection characteristic of tube 325. Thatis to say, while tube 325 has a rejection rate which increases withincreasing signal level, the second tube 326' operates with a rejectionrate which decreases with increase of signal. The result of thisautomatic compensation [feature is a more nearly constant ratio ofsignal passed to signal rejected.

The output of tube 326 goes into the third tube 327, which is of thesharp cut-off type and operates with a. fixed cut-off bias which isadjustable to the proper value by a potentiometer 365 to remove theresidual noise from the input signals. The potentiometer 365 isregulated by a fingerpiece 320 on the control panel. A condenser 3270and resistor 327d form a conventional interstage coupling for the grid327a, but the time constant of 327c-327d is very short as compared withthat of 325b325c--325d.

The tube 327, which operates beyond cut-01f, passes only those signalsthat are above the minimum voltage determined by the grid biasset-tingof potentiometer 365. This will be understood from diagram A3 ofFig. 2. where the input wave coming from tube 326 is indicated by a2 andthe fixed negative bias of tube 327 is denoted by the line b2. The tube327 does not start to conduct until the wave a2 reaches the bias levelb2 after an interval measured by the line x, the meaning of which waspreviously explained in connection with tube 325. [In other words, thethird tube 327 passes only those portions of the signal wave which arepositive beyond the fixed negative bias on grid 327a and those passed-onportions of the signal wave are positive pulses of approximatelyrectangular wave shape. The negative pulses of the signal waves, it willbe recalled, are eliminated in tubes 325 and 326. Therefore, what we arepassing through the third tube 327 are positive signal pulses ofapproximately rectangular shape, free of all deleterious noise andsubstantially uniform, as indicated in the wave diagrams A3, B3 and C3of Fig. 2.

The fourth tube 328 is a double triode used as a gridlimitingover-driven amplifier to limit the higher signals passed by the third,tube 327 in excess of the minimum signals. The tube 328 receives therectangular signal impulses from tube 327 through a differentiatingnetwork composed of a condenser 328a and a resistor 328d, which changesthe rectangular positive wave into sharp positive and negative pulses bythe charging and discharging of the condenser. This will be understoodby electrical engineers without further explanation and we need onlyrefer to diagrams A4, B4 and C4 of Fig. 3 as illustrating theapproximate shape of the differentiated wave before passing into thetube 328. The condenser charge flows to the grid 328a through a resistor328b which limits the peaks of these positive and negative pulses sothat they are shaped into a more uniform A. C. wave. The output of theleft side of tube 328 is indicated by the diagrams A5, B5 and C5, andthe output of the right side of tube 328 (that is, the final output ofthis tube) is indicated by the diagrams A6, B6 and C6.

Comparing the widely varying signal input of the first tube 325 with theoutput of the fourth tube 328 (diagrams A6-B6C6) we see the efficientoperation of the amplifier in converting transmitted signals,fluctuating over a wide range, into recording signals of relativelyuniform energy content. The final output of limiter tube 328 passesthrough a voltage divider network which contains a gain control 366 anda record level control 367 for the final tube 329. The gain control 366is a potentiometer adjusted at the factory by means of the dial 331mounted on the base of the receiver. The record level control 367 is apotentiometer regulated by the fingerpiece 321 on the control .panel toeffect a Vernier adjustment of the gain required by any particularmachine.

The final tube 329 of the recording amplifier is a conventionalbeam-power tube which amplifies the received signals to the requiredstrength. The output of tube 329 is va fairly constant A. C. voltagewhich goes into the primary coil 368 of transformer 330. The recordingvoltage induced in the secondary coil 369 of this transformer isimpressed on a recording stylus which produces marks on electrosensitivepaper in a manner well understood by facsimile engineers.

It will be apparent from the foregoing description that the presentfacsimile recording amplifier differs materially in construction andoperation from prior amplifiers used in radio receiver circuits forautomatic volume control. As is Well known, in radio receivers it isnecessary to maintain distortion at a minimum and that limits the rangeof satisfactory operation of the receiver. Furthermore the output ofradio receivers actually increases with an increase of input signal,although the rate of such increase is somewhat cut down by automaticvolume control.

This novel recording amplifier is concerned only with black and whitesignals (that is, with signal and nosignal), intermediate tones notbeing important. Therefore the amplifier had to be designed in such away as to maintain the energy content of the output recording signalsrelatively constant over a wide range in input level. In other words,distortion of the signal wave shape in this equipment is unimportant aslong as the black marks are recorded with approximately constant densityfor weak and strong signals. To cite specific figures by way ofillustration, the particular form of recording amplifier hereindisclosed was designed to operate with a signal input variation of 45decibels and the values of the condensers and resistors used in the tubecircuits were so chosen that a change of at least 40 decibels in theinput level will produce a change in the recording level of less than 2decibels. Such a small re duction of voltage on the recording styluswill not appreciably lessen the black density of the recorded mark. As aresult, weak signals passing through the amplifier are recorded withpractically the same density .as strong signals.

The time constants of the interstage couplings containing the condensers326e, 3270 and 3280 have large values.

so that the low frequency components of signals pass readily through theamplifier, which operates on frequencies of wide audio range (forexample, between 1000 and 4000 cycles per second). With respect to thenoise removing function of our amplifier, the circuits are so designedthat noise of less than half a signal amplitude will not appreciablydetract from the recorded copy.

In describing the operation of the recording amplifier reference hasbeen made to signals of difierent strengths or levels as weak, moderateand strong. It is realized that these terms are merely relative and itwould be impossible to give :any set of accurate figures applicable toevery embodiment of the invention. However, there can be given a fairlygood idea of what is regarded as weak, moderate and strong signals byconsidering what was actually done in the particular apparatus hereindescribed as illustrative of the invention. The amplifier is so designedthat the minimum input voltage on which it will operate satisfactorilyis of the order of 0.2 volt, and it was assumed that the maximum signalvoltage received would be about 40 volts. Weak signals may be said to bethose of such low voltage (in this case, up to 0.5 volt) that the firsttube 325 passes both the positive and 6 negative pulses, although aportion of the negative pulses may be compressed. T hose signals inwhich the tube 325 clips off a portion of the positive peak andeliminates a part of the negative pulse, are referred to as moderatesignals which vary, let us say, between 0.5 and 5.0 volts. Signals ofhigher voltage may be designated as strong or large signals for thepurposes of this specification. It goes without saying that the abovefigures are put forth merely as examples and do not imply anyrestriction on the scope of the invention. As for the wave diagrams ofFigs. 2 and 3, it will be understood that these are not of mathematicalaccuracy and were drawn as rough approximations of changes in the signalwaves as they pass from one tube to the next in the recording amplifier.

Further, it is evident that various modifications of the disclosedembodiment of the invention could be made without departing from thescope and spirit of the appended claims.

What is claimed is:

1. In a facsimile telegraph system employing periodic phasing pulses, arecording amplifier comprising first tube means having a control grid,screen grid and suppressor grid, first circuit means to apply receivedsignals to said first tube means, said first circuit means including aseries combination of a blocking capacitor, input resistor and grid biascapacitor connected to said control grid, a load resistor connectedbetween ground and the junction of said blocking capacitor and inputresistor, a grid leak resistor connected to ground and to said controlgrid whereby weak signals will apply a low negative bias on the firsttube means resulting in high gain and strong signals will apply a highnegative bias on the tube means resulting in low gain, said inputresistor having a low value of resistance relative to a high resistancevalue of said grid leak resistor whereby the grid bias capacitor will befully charged during the interval of each phasing pulse and thedischarge time of the capacitor is at least equal to the period betweensuccessive phasing pulses, second tube means having a control grid,screen grid and suppressor grid, second circuit means connecting theoutput of the first tube means to the input of the second tube means,said second circuit means including a series combination of a secondcapacitor and second resistor connected between the plate of the firsttube means and ground, third resistor connected to the junction of saidsecond resistor and the second capacitor and connected to the controlgrid of said second tube means, third tube means having a control grid,screen grid and suppressor grid, third circuit means interconnecting theplate of said second tube means and the control grid of the third tubemeans, said third tube means having biasing means including apotentiometer set to bias the third tube means beyond cut-off wherebyonly those portions of a signal more positive than a predeterminedamount will pass, a differentiating network connected to the plate ofsaid third tube means and ground, fourth tube means comprising a doubletriode, resistor means connecting said differentiating network to theinput of said fourth tube means, said fourth tube means operating as agrid-limiting amplifier whereby the positive and negative pulses areshaped into uniform A. C. waves.

2. In a facsimile telegraph system employing periodic phasing pulses, arecording amplifier comprising first tube means having a control grid,screen grid and suppressor grid, first circuit means to apply receivedsignals to said first tube means, said first circuit means including aseries combination of a blocking capacitor, input resistor and grid biascapacitor connected to said control grid, a load resistor connectedbetween ground and the junction of said blocking capacitor and inputresistor, a grid leak resistor connected to ground and to said controlgrid whereby weak signals will apply a low negative bias on the firsttube means resulting in high gain and strong signals will apply a highnegative bias on the tube means resulting in low gain, said inputresistor having a low value of resistance relative to a high resistancevalue of said grid leak resistor whereby the grid bias capacitor will befully charged during the interval of each phasing pulse and thedischarge time of the capacitor is at least equal to the period betweensuccessive phasing pulses, second tube means having a control grid,second circuit means connecting the output of the first tube means tothe input of the second tube means, said second circuit means includinga series combination of a second capacitor and second resistor connectedbetween the plate of the first tube means and ground, a third resistorconnected to the junction of said second resistor and the secondcapacitor and connected to the control grid of said second tube means,third tube means having a control grid, third circuit meansinterconnecting the plate of the second tube means and the control gridof the third tube means, said third tube means having biasing meansincluding a potentiometer set to bias the third tube means beyond cutoffwhereby only those portions of a signal more positive than apredetermined amount will pass, a differentiating net work connected tothe plate of said third tube means and ground, fourth tube meanscomprising a double triode, resistor means connecting saiddifferentiating network to the input of said fourth tube means, saidfourth tube means operating as a grid-limiting amplifier whereby thepositive and negative pulses are shaped into uniform A. C. waves.

References Cited in the file of this patent UNITED STATES PATENTS2,113,214 Luck Apr. 5, 1938 2,176,663 Browne et al Oct. 17, 19392,265,689 Dome Dec. 9, 1941 2,289,840 Herz July 14, 1942 2,298,657 Smithet a1. Oct. 13, 1942 2,305,842 Case Dec. 22, 1942 2,423,671 Wolff July8, 1947 2,615,943 Gouriet et a1. Oct. 28, 1952

